Relief valve system for vessels undergoing intermittent explosions



ec. 253, GS M LUDWIG 3,417,776

RELIEF VALVE SYSTEM FOR vEssELs UNDERGOING INTEEMITTENT ExPLosIoNs FiledJuly le, 1964 2 Sheets-S1169# 1 HUIIHII INVENTOR MILTON Luow/c Dec. 24,1968 M. LUDWIG y 3,417,776

RELIEF VALVE SYSTEM FOR VESSELS UNDERGOING INTERMITTENT EXPLOSIONS FiledJuly 16, 1964 2 Sheets-Sheet 2 |T`7 IL F'IG.2

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oo (1 O ,6 O lNvENToR M/LTON LUDWIG BY ff s.; F|G.3 U M United StatesPatent O 3,417 776 RELIEF VALVE SYSTEM FOR VESSELS UNDER- GOINGINTERMITTENT EXPLOSIONS Milton Ludwig, Berkeley, Calif., assignor toChevron Research Company, a corporation of Delaware Filed July 16, 1964,Ser. No. 383,195 1 Claim. (Cl. 137-5125) This invention relates topressure relief systems. yIt relates particularly to a spheridal reliefvalve system for relieving intermittent pressure changes created byexplosions within hydrocarbon-conversion vessels and the like, and hasparticular utility in alleviating pressure changes in vessels used inthe production of phthalic anhydride by the controlled oxidation ofhydrocarbons such as ortho- Xylene.

It is an object of the present invention to provide a pressure reliefsystem having: (l) high reliability; (2) high reaction and recoverysensitivity; (3) high capacity; (4) low fabrication, operating andmaintenance costs; and (5) quiet operation; by the use of a sphericalrelief valve system attached to a vessel used in an explosion-proneprocess. The valves of the type herein disclosed are provided with apressurized environment separated from the atmosphere by means of ascalable cover member also attached to the head of the vessel. By thisarrangement ultrafast relief of pressure created by explosions withinthe vessel can be achieved in a reliable economical manner `withoutdisrupting the operation of the vessel.

Explosions occur in the production of many volatile compounds but areespecially prevalent in those processes involving the oxidation andconversion of hydrocarbons at elevated temperatures, It is customary incarrying out these processes to design the conversion vessels to havepressure relief systems operative by pressure created by explosionswithin the vessel. Heretofore, designers and engineers have used eitherbursting disc or spring-loaded plate relief systems for relievingvessels used in explosionprone processes, but each type of systemsuffers from serious limitations such as excessive noise, high cost, lowreliability, slow response, and need for replacement after eachexplosion.

In accordance with the present invention, the foregoing problems areeliminated by utilizing a lightweight spherical relief valve system forvessels used in processes undergoing intermittent explosions, Inaccordance with one aspect of the invention the valving syestemcomprises a system of spherical relief valves includin-g hollow ballsarranged in valve seats in a base plate attached to or integrally formedwith an internal head of the conversion vessel. The hollow balls areheld in the valve seats by positive pressure provided by a connection tothe feed line of the process, or by a separate pressure system includinga compressor and control valves. In operation, the low inertia of thehollow balls makes it possible for the differential pressure between thelower and upper side of the valve to lift the valve very rapidly andrelieve the explosion-created pressure quickly and quietly, yetpermitting the balls to return to a normally closed position withoutunduly disrupting the operation of the conversion process in the vessel.

In another aspect of the invention, control of the pressure systemexterior of the vessel operating on the balls of the valving systemprovides high operational reliability in adjusting operating criteria asslight variations in operating pressure occur without necessitatingredesign of the relief system.

Other objects and features of the invention will become more apparentafter consideration of t-he following description of one embodiment ofthe invention taken in conjunction :with the following drawings wherein:

FIGURE l is a ow diagram, partially schematic, of a 3,417,776 PatentedDec. 24, 1968 process wherein a vessel is employed that may be adaptedwith a relief valve system of the present invention;

FIGURE 2 is a side view, partially cut away, of a vessel of a typeuseful in the process of FIGURE l illustrating a relief system forrelieving the vessel of explosioncreated pressure;

FIGURE 3 is a sectional view taken along line 3-3 of FIGURE 2;

FIGURE 4 is an enlarged detail drawing of a spherical relief valveincorporated within the relief system of FIG- URES 2-3;

FIGURE 5 is a lio-w diagram, partially schematic, of a process in whicha modification of the relief valve system of FIGURE 2 is useful; and

FIGURE 6 is an 'enlarged detail drawing of a modiiied spherical reliefvalve of the relief system used in the process of FIGURE 5.

FIGURE 1 illustrates, in iio'w diagram form, a process including avessel 10 wherein the pressurel relief system embodying the inventionmay be employed. In the vessel 10` a process such as the oxidation oforthoxylene to phthalic anhydride may be in progress. Vessel l10includes an inlet 11 and lan outlet 12. In the suggested process, theinlet 11 connects to sources of air and orthoxylene by way of feed line13. The air emanates from high-pressure blower 14 and is heated toapproximately 300 F. by preheater 15 prior to entry into the vessel. Theorthoxylene is mixed with the air in stoiohiometric proportions ofapproximately 17:1 at atomizer 2.1 from a source generally indicated at22. Referring now to FIGURES 2 and 3, after the mixture passes throughthe inlet 11 the temperature of the mixture is increased toapproximately 1050" F. rwithin reaction zone 17 of the vessel whereinthe orthoxylene is oxidized to vaporized phthalic anhydride in thepresence of a vanadium oxide catalyst. The catalyst is located in thereaction zone within tube members 25 connected to tube sheets 26 and 27.The phthalic anhydride is exhausted through outlet 12. The interior ofthe vessel is maintained at a proper reaction temperature by means of aheat exchange medium such as molten salt passing transversely throughthe vessel Iby way of inlet 28a and outlet 2-8b. The salt is confinedwithin the vessel by baf'es 29 and tube sheets 26 and 27.

Experience has shown that the mixture of air and orthoxylene sometimesaccumulates in an explosive mixture between tube sheet 26 and internalhead 16 of the vessel. If an explosion occurs within this zone, i.e.,burning of the mixture at a rapid but nite rate, the pressure risesrapidly throughout the vessel. To relieve the explosion pressure in theportion of the vessel below the internal head, a pressure relief systemis conveniently attached to the internal head 16. As discussed above,however, conventional relief systems to the atmosphere may not react-fast enough, may be too noisy, or uneconomical for effective in-plantoperations.

As ydistinguished from previously known techniques, the presentinvention permits rapid relief of closed vessels containing processessubject to intermittent explosions in an economical and quiet manner.The relief system is designated 30 in FIGURES 2 and 3 and is morespeciiically shown in FIGURE 4.

As shown in FIGURE 2, relief sytsem 30 attaches to the raised centralportion of head 16 and includes a plurality of spherical relief valves31 subjected to a pressurized gas medium, usually air, within a chamber32. The purpose of the pressurized gas is to provide a net downwardforce on the valves `during normal operation of the vessel yet asexplosion-created pressure rises within the vessel, offer noconsequential resistance to the opening yof the valves to relieve thevessel. Frustoconical cover member 33 attaches to the upper skirt of thevessel at flange 34 to form the side and end Walls of chamber 32. Theremaining side of the chamber is formed by the head 16.

Each valve 31 includes a valve cage 3S attached to the hea-d 16 inalignment with an opening 36. The valve cage acts as a guide for hollowsphere 37 and is provided with a bore and a series of slots 39 in itsside wall. The slots serve as passageways for the liow of gases from theinterior of the vessel when the hollow sphere 37 travels to an openposition as shown in phantom in FIGURE 4. Hollow sphere 37 is adapted toseat on annular seating element 38 during normal operation of thevessel. The upper surface of the ball is subjected to the pressureswithin the chamber 32. The seating element 38 is disposed againstshoulder 40 formed at the junction of counterbore 40a with opening 36and is retained in the counterbore by the threaded end 35a. of the ballcage 35. Upper surface 42 of the seating element is formed into afrustrum of a cone to provide a continuous line contact with the hollowsphere 37 when the lower portion of the latter is seated in Contact withsurface 42. A resilient gasket 43 is seated between shoulder 40 and theseating element 38 to prevent leakage during normal operation of thevessel. A guide 44 attaches to each valve `ball to prevent rotation ofthe valve ball during travel along the bore of the ball cage 35. It isnoted that diameter of the cage relative to the diameter of the ball canbe chosen to cushion upward travel of and consequently preventmechanical damage to the ball.

While configurations other than hollow spheres 37 may be used, a hollow'ball is preferred so that its lightness, with corresponding mechanicalruggedness, minimizes its inertia during operation. As an explosionoccurs, the sphere travels upward from the seating element 38 to the endwall of sphere cage 35. Inasmuch as the resistance of the gas `with thecha-mber 32 approaches zero, the time required for each sphere to liftfrom the seating element is minimum. As the explosion pressure isexhausted by way of slots 39, the sphere returns, by gravity, in afraction of a second to a position in Contact with the seating clement.

The magnitude of the pressure above the valve balls varies withoperating conditions but preferably is set as close as possible to theoperating pressure within the vessel to minimize the time for thespheres to lift from the seating elements. If the relief system is usedin conjunction with the vessel of FIGURE 1, for production of phthalicanhydride, the magnitude of the pressure is determined by the pressureat the attachment of bleed line 1S to air line 19 above inlet 11, lessthe pressure drop between the attachment of these lines and the exhaustof the vessel. In operation the pressure in the chamber above the valvesis only 2 or 3 p.s.i. above the normal pressure below the valves.Consequently the valves respond quickly to a sudden pressure rise in thechamber below valves without incurring the risk of ineffective relief ofthe vessel. Furthermore, the valve of the present invention is adaptableto more stringest applications where it may be necessary to have thechamber pressure independently controlled as by connecting the chamberto a separate source through a variable valve system. It is noted thatin a separately controlled relief pressure system, the pressure withinchamber 32 may lbe easily varied as process operating criteria occur.Hence flexibility of the system is achieved without loss in operationalreliability. In both modifications however it should also be noted thatthe chamber above the valves will be effective to attenuate the noisecreated by the release of pressure from the interior of the vessel. As aconsequence, personnel working adjacent to the conversion vessel will beless likely to be startled as the relief valves open to relieve thevessel.

A further feature of the relief system is the high exhaust capacityinstantaneously available to dissipate pressure from the vessel 10. Thecapacity of the system is achieved by the increased flow area of therelief system provided by the series of spherical valves 31, each ofwhich instantaneously opens to exhaust the explosion-created pressurewave from the vessel. It is noted that the diameter of each sphericalball is directly related to the area available to exhaust the vessel buthas an upper limitation in that the diameter may not be greater than 10inches. This is required if the sphere is to have the lightness andmechanical ruggedness needed for effective relief systems.

Tests have lbeen performed to show the reaction and recoverysensitivities of the valving system in accordance with the invention. Inthese tests a single spherical relief valve was located in a modelvessel and exposed to pressure waves created by the explosion oforthoXylene-air mixtures burning at rates from .O6 to .l second. The

results of the test are set out in full below:

Sec. Reaction time-Closed to open 0.02 Recovery time-Open to closed 1Inspection of the data illustrates that the spherical relief valve hasan exceedingly high reaction time to an incident pressure wave createdby an explosion as well as a quick recovery to normal closed position incontact with the seat element of the valve after the explosive wave hasbeen dissipated.

MODIFICATION Refinery operations often require that the pressure mediumabo-ve the relief system be completely isolated from the interior of thevessel undergoing intermittent explosions. Such a requirement is usualwhere the pressure medium above the spheres does not form an integralpart of the production process, or where small amounts of the pressuremedium leaking into the vessel can upset the conversion balance. Forsuch applications, it is desirable to provide a relief system not onlyfor an ultrafast response and recovery sensitivities but also havingmeans of isolating the interior of the vessel from the pressure mediumabove the valve balls of the relief system.

In FIGURES 5 and 6 a modification of the relief systcm previouslydescribed is shown for preventing the pressure medium above thespherical check valves from leaking into vessel S0. The relief systemincludes a series of modified spherical valves 31' connected throughmanifold 51 to a source of air pressure, usually a blower 52. Thepurpose of manifold 51 and separate feed lines 51a connected to thevalves is to separately pressurize each valve independent of theoperating pressure of the vessel. Referring now to FIGURE 6, each valveincludes a ball cage 35 threadably attached to the head of the vessel inalignment with an opening 36. The ball cage acts as a guide for sphere37 and is provided with a series of slots 39' `for exhausting thepressure wave to the atmosphere exterior of the vessel. Sphere 37' isadapted to seat on annular seating element 3S in simultaneous contactwith both a first O-ring 53 supported in the seating element and asecond O-ring 54 disposed in the side wall of valve cage 35'. FirstO-ring 53 is supported in groove 45S in the seating element below slots39 and is brought into pressure-sealing contact with an exterior portionof sphere 37. Second O-ring 54 is supported in groove 57 for-med in theside wall of valve cage 35 above slots 39 also contacting the exteriorof sphere 37 at a location above first O-ring 53. The second O-ring 54forms a check seal with the sphere along a surface defining a planepassing through the center of formation of the valve ball.

While other sealing configurations can be used, a' configuration inwhich the diameter of O-ring 53 and groove 55 are smaller than theO-ring `54 and the groove `57 is preferred. It is apparent that thesmaller area exposed to the interior pressure of the vessel when O-ring53 is in contact with sphere 37', will allow operation of the reliefsystem even though the differential operating pressure between thevessel and ball cage 35 is zero. Consequently connecting the interior ofthe vessel to the manifold above each valve, as for example, by a bleedline, will allow satisfactory operation of the system. Aside from theadvantage of lower fabrication and maintenance cost, the Voperation ofthe system in this manner will also be self-regulating. That is, changesin the operating pressure within the vessel will be vautomaticallyrellected at the manifold. Consequently, if the operating pressure ofthe process is changed during the process run, the differential pressurebetween the interior of the Vessel and each valve remains constantirrespective of the magnitude of the change in operating pressure.

The valve pictured in FIGUR-E 6 may have application apart from vessel50. For example, a valve or a series of valves may be attached to reliefports to relieve storage tanks or pipe lines if ultrafast relievingaction, self-regulation and self-recovery are required in theseapplications. However, the cvalve of FIGURE 6 cannot operate in allenvironments. The operating temperatures of the pressure medium aboveand below the O-rings 53 an-d 54 must be within the operatingcapabilities of the material forming the O-rings. `Otherwise if thetemperatures are too high, deterioration of seals between the O-ringsand the valve ball occur over the operational cycle of the valve.

While certain preferred embodiments of the invention have beenspecifically disclosed, it should be understood that the invention isnot limited thereto as many variations will be readily apparent to thoseskilled in the art and the invention is to be given its broadestpossible interpretation within the terms of the following claim.

What is claimed is:

1. In a relief system for an explosion-prone vessel having a head, atleast one inlet and one outlet, a central reaction zone and at least onereactant within said zone, the combination of support means having aseries of openings ytherethrough in operative contact with said centralreaction zone, a plurality of spherical relief valves attached to saidsupport means in cooperative alignment with said openings, each of saidvalves including a cylindrical valve cage in alignment with onej of saidopenings, a separate seating element attached within each of saidopenings wherein said valve cage and said seating element have surfacesin cooperating contact within said opening, a hollow sphere slidablewithin said valve cage and having a first arcuate portion in operablecontact with said seating element to pro-vide a seal and a secondarcuate portion remote therefrom, and a source of pressurized 4gascontrollably connected to each of said valves for providing gas pressureover said second portion of said sphere to maintain a substantiallyconstant pressure differential between said rst and second portions ofsaid sphere and maintain said sphere in contact with said seatingelement except in response to explosion-created increases in pressure insaid reaction zone of said vessel.

References Cited UNITED STATES PATENTS 1,054,794 3/1913 Riegel 137-5121,211,283 1/1917 Butler 137-533.13 X 1,666,962 4/1928 Dennis et al.137-533.13 X 2,292,946 8/ 1942 Karig 165-11 X 2,770,255l 11/1956 Goddard137-529 OTHER REFERENCES Glattli, Three-Terminal Ball Elements, IBMTechnical Disclosure Bulletin, vol. 6, No. 2, p. 28; July 1963.

WILLIAM F. ODEA, Primary Examiner. D. I. ZOBKIW, Assistant Examiner.

U.S. Cl. X.R.

1. IN A RELIEF SYSTEM FOR AN EXPLOSION-PRONE VESSEL HAVING A HEAD, ATLEAST ONE INLET AND ONE OUTLET, A CENTRAL REACTION ZONE AND AT LEAST ONEREACTANT WITHIN SAID ZONE, THE COMBINATION OF SUPPORT MEANS HAVING ASERIES OF OPENINGS THERETHROUGH IN OPERATIVE CONTACT WITH SAID CENTRALREACTION ZONE, A PLURALITY OF SPHERICAL RELIEF VALVES ATTACHED TO SAIDSUPPORT MEANS IN COOPERATIVE ALIGNMENT WITH SAID OPENINGS, EACH OF SAIDVALVES INCLUDING A CYLINDRICAL VALVE CAGE IN ALIGNMENT WITH ONE OF SAIDOPENINGS, A SEPARATE SEATING ELEMENT ATTACHED WITHIN EACH OF SAIDOPENINGS WHEREIN SAID VALVE CAGE AND SAID SEATING ELEMENT HAVE SURFACESIN COOPERATING CONTACT WITHIN SAID OPENING, A HOLLOW SPHERE SLIDABLEWITHIN SAID VALVE CAGE AND HAVING A FIRST ARCUATE PORTION IN OPERABLECONTACT WITH SAID SEATING ELEMENT TO PROVIDE A SEAL AND A SECOND ARCUATEPORTION REMOTE THEREFROM, AND A SOURCE OF PRESSURIZED GAS CONTROLLABLYCONNECTED TO EACH OF SAID VALVE FOR PROVIDING GAS PRESSURE OVER SAIDSECOND PORTION OF SAID SPHERE TO MAINTAIN A SUBSTANTIALLY CONSTANTPRESSURE DIFFERENTIAL BETWEEN SAID FIRST AND SECOND CONSTANT PRESSUREDIFFERENTIAL BETWEEN SAID SPHERE IN CONTACT WITH SAID SEATING ELEMENTEXCEPT IN RESPONSE TO EXPLOSION-CREATED INCREASES IN PRESSURE IN SAIDREACTION ZONE OF SAID VESSEL.